HN-66000 is a proprietary pharmaceutical formulation of mutant diphthieria toxin and human transferrin that is being developed as a potential treatment for malignant brain tumors. The results from Phase I trial sponsored by the NIH suggest that the drug produces tumor responses in patients with malignant brain tumors refractory to conventional therapy without severe neurologic or systemic toxicity. The encouraging results warrant an expanded evaluation in a phase II trail.